The invention relates to switched network telecommunications. More particularly, the invention relates to a sensitive current sensing relay which detects the presence or absence of a loop current in a switched network modem interface.
In switched network modems, it is often necessary to detect the presence or absence of loop current, for example to determine whether a valid call is still in process, or whether an ancillary device such as a phone is off-hook. In switched networks, the subscriber loop is generally energized with -48 volts DC from a central office or PBX. This voltage causes loop current to flow when a valid call is in progress and "term set" (e.g., phone) is off-hook. There are generally two known ways of detecting the presence or absence of loop current: either by an LED OPTO coupler or by a current sensitive relay. Regulatory requirements as to the types of equipment which may be connected to some public telecommunications networks (e.g., international networks) have in effect mandated the use of relays rather than OPTO couplers because of the DC voltage drop allowed across the sensor. Prior art FIG. 1 shows a simplified schematic diagram of a state of the art current sensing relay in a switched network line interface.
The switched network (VF) line modem interface of prior art FIG. 1 includes port A for connecting to the network and port B for coupling a device, e.g. a phone, for use with the network. The interface and the device coupled through it to the network are generally protected by resistors R.sub.1, R.sub.2, fuse F and varistor MOV. Resistors R.sub.1 and R.sub.2 are typically 5.OMEGA., 3 watt, wire wound resistors which are used in connection with the varistor so as to prevent damage to both the interface and to the device coupled to the network via the interface from voltage and current surges due to e.g., lightning. Also seen in FIG. 1 is an "off-hook" relay OH which couples to the analog portion of the modem which includes DC hold circuit HOLD, capacitor C, transformer T, and resistor R.sub.T.
As seen in FIG. 1, a current sensing relay K1 is arranged in series between ports A and B. The current sensing relay K1 is effectively a reed relay capsule 2 having reed contacts 4 and 6. The reed relay is coupled to the core 100 of the modem which includes a demodulator (not shown), a data pump (not shown), etc., as is known in the art. The core is also typically connected to a ring detector RING 110 as shown in phantom. Regardless, with the current sensing relay arranged as shown, current flowing through the interface from port A is sensed. For example, when the device coupled to port B is "off-hook", loop current flows through the interface. The relay K1 detects this loop current and signals the modem that the port B device is already "off-hook". Also, if a correct signal is provided at port A (typically as detected by ring detector RING), the modem goes off-hook and closes switch OH. When that happens, current flows through the coil of the read relay K1, and the reed relay contacts 4 and 6 close. As long as the call is not terminated, contacts 4 and 6 will be closed. However, when the call is terminated, contacts 4 and 6 open, and this fact is determined by the modem core 100.
Relay K1 is typically a reed relay of the type shown in prior art FIG. 2. It generally comprises a reed capsule 2 containing reed contacts 4, 6 which open or close in response to a magnetic field. Capsule 2 is surrounded by a coil bobbin 8 containing a wound coil 10 which typically has approximately 800-1000 turns. A magnetic stator shield 12, typically a mu-metal foil (high permeability magnetic foil) of 2 mils thickness, is wrapped around coil 10. Relay K1, as shown in the circuit of FIG. 1, has a DC resistance R.sub.K1 typically of 10.OMEGA., an operating inductance L.sub.K1 of approximately 3.5 millihenries and an impedance .vertline.Z.sub.LK1 .vertline. at 4 KHz of approximately 88.OMEGA. (4 KHz being the bandwidth of a single voice channel in a switched network). The total DC resistance (R.sub.DC) between ports A and B is of considerable importance both for performance and for regulatory compliance. Generally lower resistances are preferred, and it is preferred that R.sub.DC .ltoreq.10.OMEGA.. In the prior art example of FIG. 1, the total R.sub.DC is 20.OMEGA. (R.sub.1 +R.sub.2 +R.sub.K1).
The high inductance of the relay K1 is problematic because of its impedance at high frequencies (impedance being a product of inductance and frequency: Z=2.pi. fL). In order to reduce the impedance of relay K1 during communication, it is necessary to add electrolytic bypass capacitors C.sub.1 and C.sub.2 as shown in FIG. 1. For very high speed modems, C.sub.1 and C.sub.2 are typically chosen to be 220 microfarads each, resulting in a total capacitance of 110 microfarads: C.sub.TOTAL =(C.sub.1 C.sub.2)/(C.sub.1 +C.sub.2). In the circuit shown in FIG. 1, the bypass capacitors change the impedance across K1 (Z.sub.K1) according to the equation Z.sub.K1 =(Z.sub.C)(Z.sub.R +Z.sub.L)/(Z.sub.C +Z.sub.R +Z.sub.L) where Z.sub.C =1/(jwC), Z.sub.L =jwL, Z.sub.R =R.sub.K1, j=v-1, and w is frequency in radians per second. Thus at 4 KHz, Z.sub.C is approximately 0.362.OMEGA., Z.sub.LK1 is approximately 88.OMEGA. and Z.sub.RK1 is always 10.OMEGA.. Therefore, at 4 KHz, the total impedance across relay K1 with the bypass capacitors is approximately 0.361.OMEGA..
It will be appreciated by those skilled in the art that while the bypass capacitors serve the useful function of reducing the impedance of the relay at higher frequencies, the capacitors are relatively expensive and take up additional space in the circuit. In addition, these capacitors do nothing to reduce the total DC impedance of the circuit which is still greater than the preferred 10.OMEGA.. Also, it will be appreciated that the addition of the capacitors in parallel with the inductor L.sub.K1 causes a parallel tuned circuit to be formed so that at a particular low frequency f=1/2.pi.(C.sub.T L.sub.K1).sup.0.5, the impedance of the relay can be quite high. Thus, the capacitance C.sub.T must be made large enough so that the resonant frequency is lower than the frequency of interest for the circuit.